Systems and methods for corneal surface ablation to correct hyperopia

ABSTRACT

Systems, methods and apparatus for performing selective ablation of a corneal surface of an eye to effect a desired corneal shape, particularly for correcting a hyperopic/astigmatic condition by laser sculpting the corneal surface to increase its curvature. In one aspect of the invention, a method includes the steps of directing a laser beam onto a corneal surface of an eye, and changing the corneal surface from an initial curvature having hyperopic and astigmatic optical properties to a subsequent curvature having correctively improved optical properties. Thus, the curvature of the anterior corneal surface is increased to correct hyperopia, while cylindrical volumetric sculpting of the corneal tissue is performed to correct the astigmatism. The hyperopic and astigmatic corrections are preferably performed by establishing an optical correction zone on the anterior corneal surface of the eye, and directing a laser beam through a variable aperture element designed to produce a rectangular ablation (i.e., cylindrical correction) on a portion of the optical correction zone. The laser beam is then displaced by selected amounts across the optical correction zone to produce a series of rectangular ablations on the correction zone that increases the curvature of the corneal surface to correct the hyperopic refractive error.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] The present application is a continuation application of U.S.patent application Ser. No. 09/379,372 filed Aug. 23, 1999, which is acontinuation-in-part of U.S. patent application Ser. No. 08/906,020filed on Aug. 5, 1997, which is a continuation-in-part of U.S. patentapplication Ser. No. 08/968,380 filed on Nov. 12, 1997, which is acontinuation of U.S. patent application Ser. No. 08/058,599 filed on May7, 1993, the full disclosures of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

[0002] The present invention is directed to systems, methods andapparatus for performing selective ablation of a corneal surface of aneye to effect a desired corneal shape. In particular, the presentinvention is directed to methods for correcting a hyperopic condition ofthe eye by laser sculpting the corneal surface to increase itscurvature.

[0003] Ultraviolet and infrared laser based systems and methods areknown for enabling ophthalmological surgery on the external surface ofthe cornea in order to correct vision defects. These proceduresgenerally employ an ultraviolet or infrared laser to remove amicroscopic layer of an anterior stromal tissue from the cornea to alterits refractive power. In ultraviolet laser ablation procedures, theradiation ablates corneal tissue in a photodecomposition that does notcause thermal damage to adjacent and underlying tissue. Molecules at theirradiated surface are broken into smaller volatile fragments withoutheating the remaining substrate; the mechanism of the ablation isphotochemical, i.e. the direct breaking of intermolecular bonds. Theablation removes a layer of the stroma to change its contour for variouspurposes, such as correcting myopia, hyperopia, and astigmatism. Suchsystems and methods are disclosed in the following U.S. patents andpatent applications, the disclosures of which are hereby incorporated byreference: U.S. Pat. No. 4,665,913 issued May 19, 1987 for “METHOD FOROPHTHALMOLOGICAL SURGERY”; U.S. Pat. No. 4,669,466 issued Jun. 2, 1987for “METHOD AND APPARATUS FOR ANALYSIS AND CORRECTION OF ABNORMALREFRACTIVE ERRORS OF THE EYE”; U.S. Pat. No. 4,732,148 issued Mar. 22,1988 for “METHOD FOR PERFORMING OPHTHALMIC LASER SURGERY”; U.S. Pat. No.4,770,172 issued Sep. 13, 1988 for “METHOD OF LASER-SCULPTURE OF THEOPTICALLY USED PORTION OF THE CORNEA”; U.S. Pat. No. 4,773,414 issuedSep. 27, 1988 for “METHOD OF LASER-SCULPTURE OF THE OPTICALLY USEDPORTION OF THE CORNEA”; U.S. Patent application Ser. No. 109,812 filedOct. 16, 1987 for “LASER SURGERY METHOD AND APPARATUS”; U.S. Pat. No.5,163,934 issued Nov. 17, 1992 for “PHOTOREFRACTIVE KERATECTOMY”; U.S.patent application Ser. No. 08/368,799, filed Jan. 4, 1995 for “METHODAND APPARATUS FOR TEMPORAL AND SPATIAL BEAM INTEGRATION”; U.S. patentapplication Ser. No. 08/138,552, filed Oct. 15,1993 for “METHOD ANDAPPARATUS FOR COMBINED CYLINDRICAL AND SPHERICAL EYE CORRECTIONS”; andU.S. patent application Ser. No. 08/058,599, filed May 7, 1993 for“METHOD AND SYSTEM FOR LASER TREATMENT OF REFRACTIVE ERRORS USING OFFSETIMAGING”.

[0004] The technique for increasing the curvature of the corneal surfacefor hyperopia error correction involves selectively varying the area ofthe cornea exposed to the laser beam radiation to produce an essentiallyspherical surface profile of increased curvature. This selectivevariation of the irradiated area may be accomplished in a variety ofways. For example, U.S. Pat. No. 4,665,913 cited above discloses thetechnique of scanning the region of the corneal surface to be ablatedwith a laser beam having a relatively small cross-sectional area(compared to the optical zone to be ablated) in such a manner that thedepth of corneal removal increases with distance from the intendedcenter of ablation. This is achieved by scanning the beam more timesover the deeper regions than the shallower regions. As pointed out inU.S. Pat. No. 5,163,934, such ablations tend to be rougher than areaablations. The result is a new substantially spherical profile for theanterior corneal surface with maximum depth of cut at the extreme outerboundary of the optical zone. Another technique disclosed in theabove-cited U.S. Pat. No. 4,732,148 employs a rotatable mask having aplurality of elliptical annular apertures which are progressivelyinserted into the laser beam path to provide progressive shaping of thelaser beam in order to achieve the desired profile.

[0005] One of the major difficulties encountered in the application oflaser surgery techniques to effect hyperopic refractive errorcorrections lies in the nature of the boundary between the optical zoneand the untreated area. Since the anterior surface of the cornea issculpted during the process to have an increased curvature, the maximumdepth of cut necessarily occurs at the outer boundary of the opticalzone. The generally annular region between this outer boundary and theadjacent untreated anterior surface portion of the cornea typicallyexhibits steep walls after the completion of the photoablationprocedure. After the surgery, the tendency of the eye is to eliminatethese steep walls by stimulated healing response involving concurrentepithelial cell growth and stromal remodeling by the deposition ofcollagen, which results in corneal smoothing by filling in tissue in thesteep walled region. This natural healing response acts to eliminate thediscontinuity, resulting in a buildup of tissue in the steep walledregion and over the outer portion of the optical zone. This naturalphenomenon, sometimes termed the “hyperopic shift” in phototherapeutickeratectomy, causes a lack of precision for a given surgical procedureand diminished predictability, which tend to counteract the beneficialeffects of the refractive correction procedure and thereby reduce thedesirability of the procedure to the prospective patient.

[0006] In some patients, there are both hyperopia and astigmatismdefects in the same eye, requiring correction of both errors in order toimprove vision. Astigmatic conditions are typically caused by acylindrical component of curvature departing from the otherwisegenerally spherical curvature of the surface of the cornea. Astigmaticconditions are usually corrected by effecting cylindrical ablation aboutthe axis of cylindrical curvature of the eye. These cylindricalablations tend to increase the sharp transitions in the cornea at theextreme ends of the sculpted area.

[0007] What is needed in the field of ophthahnological surgery,therefore, are systems and methods for correcting both hyperopia andastigmatism of the eye by laser removal of the corneal surface. It wouldbe particularly desirable to perform such hyperopia and astigmatismcorrections without generating steep walls in the region between theouter boundary of the optical zone and the adjacent untreated anteriorsurface portion of the cornea.

SUMMARY OF THE INVENTION

[0008] The present invention is directed to systems, methods andapparatus for performing selective ablation of a corneal surface of aneye to effect a desired corneal shape, such as for correcting ahyperopic condition by laser sculpting the corneal surface to increaseits curvature. The present invention is particularly useful forcorrecting hyperopic conditions with a cylindrical component ofcurvature (i.e., astigmatism). However, it will be appreciated that thesystems and methods of the present invention can be applied equally wellto the correction of other refractive procedures, such as myopia,irregular astigmatism, or combinations thereof.

[0009] In one aspect of the invention, a method includes the steps ofdirecting a laser beam onto a corneal surface of an eye, and changingthe corneal surface from an initial curvature having hyperopic andastigmatic optical properties to a subsequent curvature havingcorrectively improved optical properties. Thus, the curvature of theanterior corneal surface is increased to correct hyperopia, whilecylindrical volumetric sculpting of the corneal tissue is performed tocorrect the astigmatism. The hyperopic and astigmatic corrections arepreferably performed by establishing an optical correction zone on theanterior corneal surface of the eye in which the desired refractivecorrection is to be effected, and an annular transition zone around theoptical correction zone. A laser beam is directed through a variableaperture element that is designed to generate a profiled beam with agenerally rectangular shape on the cornea (i.e., cylindricalcorrection). The profiled beam is directed onto the corneal surface anddisplaced by selected amounts across the optical correction zone toproduce a series of rectangular ablations on the correction zone. Thelocations of the rectangular ablations on the optical correction zoneare selected to increase the curvature of the corneal surface to correctthe hyperopic refractive error. The angle of the rectangular ablationsare determined by the axis of the desired cylindrical correction.

[0010] The technique for increasing the curvature of the corneal surfacefor hyperopia error correction involves selectively varying the area ofthe cornea exposed to the laser beam radiation to produce a surfaceprofile of increased curvature. Thus, the rectangular ablationsgenerated by the profiled beam are displaced across the cornea such thatthe depth of corneal removal increases with distance from the intendedcenter of ablation, or the central axis of the optical correction zone.In one embodiment, the rectangular ablations are sized and displacedsuch that the outer edge of the optical correction zone (which is theportion that should receive the deepest corneal removal) will besubjected to a substantial portion (if not all) of the rectangularablations. In addition, the central portion of the optical correctionzone (which is desirably the portion that receives the least amount ofcorneal removal) receives the least amount of the ablations. Theintermediate areas of the optical correction zone will receive anappropriate amount of rectangular ablations such that the cornealsurface curvature increases in the radially outward direction to correctfor hyperopia.

[0011] In a preferred implementation of the method, the laser beampasses through a variable width slit and a variable diameter diaphragmto create a profiled beam that is imaged onto the corneal surface. Theslit width is varied in conjunction with the beam displacement toprovide a surface profile of increased curvature within the opticalcorrection zone, as discussed above. The diaphragm is maintained at alarge enough diameter to minimize its effect on the optical correctionzone. In addition, the variable diaphragm is varied in selected amountsto smooth the sharp transitions at the ends of the cylindricalcorrections. In an exemplary embodiment, the diaphragm decreases indiameter as the laser beam is displaced radially outward from a centralaxis of the correction zone, and increases in diameter as the laser beamis displaced radially inward toward the central axis. This provides amore gradual sloping of the corneal surface to eliminate the sharpdiscontinuity between the outer edge of the optical zone and the edge ofthe untreated area.

[0012] The rectangular ablations or cylindrical corrections may becreated and displaced across the correction zone in a variety ofdifferent manners. In one embodiment, the laser beam passes through thevariable aperture element to form a profiled beam that is imaged ontothe cornea with an imaging lens positioned between the laser and theeye. The image of the profiled beam is displaced across the opticalcorrection zone by first locating the lens at a starting position,pulsing the laser and then displacing the lens to a subsequent position,which is preferably the starting position plus a predeterminedincremental amount. In other embodiments, the profiled beam may bescanned across the cornea with rotating mirrors (e.g., galvanometers),rotating prisms, or the like. Alternatively, the profiled beam may bedisplaced by moving the position of the variable aperture element. Inthis embodiment, the beam will be sized to cover the entire opticalcorrection zone, and the variable aperture element will be sized todisplace the beam across this zone.

[0013] For a fuller understanding of the nature and advantages of theinvention, reference should be had to the ensuing detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a block diagram of an ophthalmological surgery systemfor incorporating the invention;

[0015]FIG. 2 is a schematic plan view illustrating a movable slit andvariable diameter aperture used in the system of FIG. 1;

[0016] FIGS. 3A-3C are schematic views showing the ablation geometry forthe aperture of FIG. 2;

[0017]FIG. 4 is a schematic view of delivery system optics of thesurgery system of FIG. 1;

[0018]FIG. 5 is a top plan view of an image offset control unit of theinvention, with the top annular portion removed; and

[0019]FIG. 6 is a side sectional view taken along lines 5-5 of FIG. 5.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

[0020] The present invention is directed to systems, methods andapparatus for performing selective ablation of a corneal surface of apatient's eye to effect a desired corneal shape. In a specificimplementation, methods are provided for correcting a hyperopiccondition by laser sculpting the corneal surface to increase itscurvature. The present invention is particularly useful for correctinghyperopic conditions with a cylindrical component of curvature (i.e.,astigmatism), while also smoothing the transition zone between theoptical correction zone and the remainder of the cornea. Forconvenience, the remaining disclosure will be directed specifically tosystems and methods for the correction of hyperopic and astigmaticrefractive errors. However, it will be appreciated that the systems andmethods of the present invention can be applied equally well to thecorrection of other refractive procedures, such as myopia, irregularastigmatism or combinations thereof.

[0021]FIG. 1 illustrates a block diagram of a representativeophthalmological surgery system for incorporating the invention. Asshown, a laser surgery system 20 includes a computer 21, such as apersonal computer work station or other conventional arrangements. Thesubcomponents of laser surgery system 20 are known components andpreferably comprise the elements of the VISX STAR Excimer Laser System™,which is commercially available from VISX, Incorporated of Santa Clara,Calif. Thus, the laser surgery system 20 includes a plurality of sensorsgenerally designated with reference numeral 22 which produce feedbacksignals from the movable mechanical and optical components in the laseroptical system, such as the elements driven by an iris motor 23, animage rotator 24, an astigmatism motor 25, an astigmatism angle motor26, an image lens motor 12 and an image lens rotation motor 10. Thefeedback signals from sensors 22 are provided via appropriate signalconductors to the computer 21. The computer controls the operation ofthe motor drivers generally designated with reference numeral 27 foroperating the elements 10, 12 and 23-26. In addition, computer 21controls the operation of the Excimer laser 28, which is preferably anargon-fluorine laser with a 193 nanometer wavelength output designed toprovide feedback stabilized fluence of 160 mjoules per cm2 at the corneaof the patient's eye 30 via the delivery system optics generallydesignated with reference numeral 29 and shown in FIG. 4. Otherancillary components of the laser surgery system 20 which are notnecessary to an understanding of the invention, such as a highresolution microscope, a video monitor for the microscope, a patient eyeretention system, and an ablation effluent evacuator/filter, as well asthe gas delivery system, have been omitted to avoid prolixity.Similarly, the keyboard, display, and conventional PC subsystemcomponents (e.g., flexible and hard disk drives, memory boards and thelike) have been omitted from the depiction of the PC work station 21.Further details of suitable system for performing a laser ablationprocedure can be found in commonly assigned U.S. Pat. Nos. 4,665,913,4,669,466, 4,732,148, 4,770,172, 4,773,414, 5,207,668, 5,108,388,5,219,343, 5,646,791 and 5,163,934, the complete disclosures of whichare hereby incorporated herein by reference.

[0022] The iris motor 23 is used to control the diameter of a variablediameter iris schematically depicted in FIG. 2. The astigmatism motor 25is used to control the separation distance between a pair of cylinderblades 35, 36 which are mounted on a platform 38 for bi-directionaltranslational motion in the direction of arrows 40, 41. Platform 38 isrotatably mounted on a second platform (not illustrated) and isrotationally driven by astigmatism angle motor 26 in a conventional wayin order to enable alignment of the slit axis (illustrated in a verticalorientation in FIG. 2) with the appropriate coordinate axes of thepatient's eye. Iris 32 is driven by iris motor 23 in a known way tochange the diameter of the iris opening from a fully opened position(the position illustrated in FIG. 2) to a fully closed position in whichthe aperture is closed to a minimum diameter of 0.8 mm. It is understoodthat the variable diameter iris 32 and the cylinder blades 35, 36 arepositioned with respect to the output of laser 28 in such a manner sothat a profiled beam shape is imaged onto the corneal surface of thepatient's eye 30. For the purpose of this application, it may be assumedthat iris 32 and cylinder blades 35, 36 are part of the delivery systemoptics subunit 29 shown in FIG. 1.

[0023] Of course, it should be understood that the laser beam may beprofiled in a variety of conventional or non-conventional manners otherthan that described above. For example, rotating masks, ablatablemembranes and/or prisms may be used to image the laser beam rather thanthe variable apertures described above.

[0024] The system of FIGS. 1 and 2 may be used according to theinvention to effect hyperopic refractive error corrections to theanterior surface of the cornea. In addition, the methods of the presentinvention provide a smooth transition zone between the outer edge of theoptical zone and the untreated surface of the cornea. With reference toFIGS. 5 and 6, an imaging lens 51 may be laterally offset or displacedfrom a central axis by a variable amount in the manner set forth morefully below. Lens 51 preferably comprises the existing imaging lensfound in the delivery system optics 29 of the FIG. 1 system which aredescribed more fully below. The image lens motor 12 is used to translatethe lens 51 relative to the central axis and the image lens rotation 10is used to rotate the lens 51 about the central axis. Displacing lens 51by translating the lens in a radial direction off the central axis,which may or may not correspond to the laser beam axis, displaces theimage of the aperture in a related manner. As discussed in more detailbelow, lens 51 may be displaced such that the image of the aperture isdisplaced across the optical correction zone to effect a series ofrectangular ablations (i.e., cylindrical corrections) across the opticalcorrection zone.

[0025] Of course, it will be recognized that the laser beam may bedisplaced or scanned across the optical correction zone with scanningelements other than the displaceable imaging lens described above. Forexample, the profiled beam may be scanned across the cornea withrotating mirrors (e.g., galvanometers), rotating prisms, or the like.Alternatively, the profiled beam may be displaced by changing the sizeof the iris 32 and cylinder blades 35, 36. In this embodiment, the beamwill preferably be sized to cover the entire optical correction zone,and the cylinder blades 35, 36 will be independently movable so that theposition of the image can be displaced across the cornea (e.g., bymoving a single cylinder blade, or by moving both blades).

[0026]FIGS. 3A and 3B illustrate the method of the present invention forcorrecting hyperopic and astigmatic refractive errors. As shown, anoptical correction zone 60 and an annular transition zone 62 areestablished on the corneal surface of the patient's eye. In thesefigures, the intended optical zone is the central region bounded bycircle 61 and the intended transition zone is the annular region boundedby circles 61 and 63. Depending on the nature of the desired opticalcorrection, optical correction zone 60 may or may not be centered on thecenter of the pupil or on the apex of the anterior corneal surface. Thecorrection zone will typically have a radius R₃ of about 2 to 3 mm andtransition zone 62 will have an outside radius of about 3 to 5 mm. Asshown, transition zone 62 may have an elliptical shape, or it may begenerally circular, depending on the desired optical correction.

[0027] Referring to FIG. 3B, R2 represents the half width of the slitbetween blades 35, 36, S represents the width of the slit between blades35, 36, R₁ represents the radius of the iris 32, I is the diameter ofthe iris 32 and E_(L) is the edge length of the blades 35, 36 which isestablished by the diameter of the iris 32. As shown in FIG. 3C, R₃ isthe radius of the optical correction zone, C_(L) is the half length ofthe optical correction zone and 0 represents the radial offset of thecenter of the image of the slit aperture relative to the center ofoptical correction zone 60. The radial offset 0 will increase as theimaging lens 51 is displaced away from the central axis and the halflength of the optical correction zone C_(L) will decrease as therectangular ablations 80 move radially outward.

[0028] In the preferred embodiment, the laser beam will be profiled suchthat it provides a cylindrical correction with little to no sphericalcomponent within the optical correction zone. Thus, the curvature of theanterior corneal surface is increased to correct hyperopia, whilecylindrical volumetric sculpting of the corneal tissue is performed tocorrect the astigmatism. The hyperopia cylinder surface is preferablycreated by using the offset mechanism to place a series of slit-shapedor generally rectangular ablations 80 over the optical correction zoneof the eye, as shown in FIG. 3A. Thus, the rectangular ablations 80 aredisplaced across the cornea such that the depth of corneal removalincreases with distance from the intended center of ablation, or thecentral axis of the optical correction zone 60. According to the presentinvention, the rectangular ablations 80 are sized and displaced suchthat the outer boundary 61 of the optical correction zone 60 (which isthe portion that should receive the deepest corneal removal) will besubjected to a substantial portion (if not all) of the rectangularablations 80. On the other hand, the rectangular ablations 80 are sizedand displaced such that the central portion of the optical correctionzone 60 (which is desirably the portion that receives the least amountof corneal removal) receives a small portion (e.g., one or zero) of theablations.

[0029] The profiled beam may start at one side of the correction zone60, and be displaced across the correction zone 60 to the other side.Alternatively, the profiled beam may start towards the center of thecorrection zone 60 (actually slightly offset from center as shown inFIG. 3A) and be displaced radially outward to place a series ofcylindrical ablations 80 over one half of the eye. In this embodiment,the profiled beam will then be placed in the center of the correctionzone (actually displaced in the opposite direction from center), anddisplaced radially outward in the opposite direction to cover the otherhalf of the eye.

[0030] The slit width between cylinder blades 35, 36 and the irisdiameter are preferably varied as the laser beam is displaced across theoptical correction zone to smooth the surface of the transition zone.For hyperopic astigmatic corrections, the iris is maintained at a largeenough diameter to minimize the effect of the aperture on the opticalcorrection zone. For hyperopia with some spherical components, thespherical correction will preferably occur before or after thecylindrical corrections.

[0031] For a hyperopic dioptric correction of a given fixed value, thesequencing of the aperture is done in such a manner as to satisfy thehyperopic lens equations described in “Photorefractive Keratectomy: Atechnique for laser refractive surgery” authored by Munnerlyn et al., J.Cataract Refract. Surg. Vol. 14, pages 46-52 (Jan., 1988), thedisclosure of which is hereby incorporated by reference. A fixed valueof the dioptric correction is used to generate the cut profile c(r). Thecut profile is given by the equation:

c(r)=−100*(R ₁ −R ₂ −{square root}{square root over (R₁ ²−y²)}+{squareroot}{square root over ( R ₂ ²−y²))}

[0032] where R₁ is the initial radius of curvature, R₂ is the finalradius of curvature and y is the distance from the center of the opticalcorrection zone 60. The sequence of aperture dimensions is created bycontrol of the diameter of iris 32 and the width of cylinder blades 35,36 throughout the surgical procedure. The sequence of aperturedimensions may also be tailored to accommodate variations in the profileof the laser beam.

[0033] After the initial slit shape has been ablated on the cornealsurface, the image of the aperture is displaced or scanned over theanterior surface of the cornea to selectively ablate the entirecorrection zone. While several different scanning sequences arepossible, the following sequence has been actually implemented witheffective results. The position of the inner edge E₁ of the slit shapefor a particular pulse is determined by the hyperopia depth calculationsof Munnerlyn as discussed above. A binary search of the radius isperformed to determine the radius from the center of the correction zonewhere the depth of that radius is equal to the depth for the pulsenumber of the treatment. The inner edge position of the cylinder blades35, 36 is generally equal to the offset O minus the slit radius R₂ andthe outer edge position of the blades is equal to the offset O plus theslit radius R₂.

[0034] In the example shown in FIGS. 3A and 3B, the initial values ofradial offset O, iris diameter I and slit width S are preferablyselected so that the inner edge E₁ of blade 35 is initially coincidentwith the central axis of the optical correction zone 60, and the outeredge E₂ of blade 35 is initially located such that a portion of outeredge E₂ is substantially coincident with the outer boundary 61 ofoptical correction zone 60. The inner edge E₁ of blade 35 is positionedto create the exact curve on the eye to create the desired cylindricalcorrection. The iris diameter I is selected such that the ends 70 of theinner edge E₁ fall outside of the correction zone boundary 61, and theends of outer edge E₂ fall inside of the outer boundary 62 of thetransition zone 62. The iris diameter I should always be large enoughsuch that the edge length E_(L) of the slit shape is greater than thecorrected length (C_(L)×2) to generate the correct cylindricalrefraction in the optical correction zone.

[0035] Once the inside edge of each slit shape is found, the slit widthS is calculated. The slit width S determines the position of the outsideedge of the slit shape. Generally, the slit width S is dependent on theinside edge E₁ and the diameters of the correction and transition zones60, 62. The initial slit width S will be calculated such that theinitial outside edge E₂ is slightly outside of the outer boundary 61 ofthe optical correction zone. Thus, the outside edge start position isequal to the correction radius plus a correction margin C_(m) or:

E ₂ =R ₃ +C _(m)

[0036] The correction margin smoothes the transition between thecorrection zone and the transition zone. The outside edge end positionE₄ is preferably located at some margin A_(m) inside the outer boundary62 of the transition zone. Thus,

E ₄=Outer boundary diameter+A _(m)

[0037] The outside edge position (O_(EP)) at any point during theprocedure is generally found by:

O _(EP)=(((OutsideEdgeEndPos)−OutsideEdgeStartPos)/CorrectionDia/2))*I_(EP))+OutsideEdgeStartPos

[0038] wherein I_(EP) is the inside edge position.

[0039] The offset position O of each slit shape is preferably determinedby the slit width S and the inner edge E₁ position. Thus:

O=E ₁ +S/2

[0040] The iris diameter I is preferably set such that the outsidecomers 72 of the slit shape are anchored at the outer boundary 63 of thetransition zone 62. Thus, the iris diameter I will be reduced as theprofiled beam is displaced radially outward (see FIG. 3A). If thiscannot be achieved, the iris diameter I is set to its maximum valuewhich will generally leave the outside comers 72 of the slit shapewithin the transition zone. Reducing the iris diameter as the beam movesoutward provides a smoothing of the transition zone 62.

IrisDiameter={square root}{square root over (I ² _(EP)+AblationDia² *O ²_(EP)−2*I _(EP) *O _(EP))}

IrisDiameter=min(MaximumIrisDia,IrisDiameter)

[0041] Thus, laser 28 is pulsed, and platform 38 and lens 51 aredisplaced to a successive position radially displaced from the previousposition by the equations described above. The laser is again pulsed,platform 38 and lens 51 are again displaced, the laser is again pulsed,etc. This process continues until the entire correction zone 60 has beencovered in incremental steps (either with one pass over the entirecorrection zone, two passes, each over half of the zone as shown in FIG.3A, or a plurality of passes, each over a section of the optical zone).

[0042] Of course, it will be recognized that the rectangular ablationsmay be scanned or displaced across the optical correction zone in avariety of manners other than that described above. For example, therectangular ablations may begin at one side of the optical correctionzone 60 within the annular transition zone 62 (e.g., with an inner bladeedge E₃ and an outer blade edge E₄, as shown in FIG. 3A). In thisembodiment, the imaging lens is displaced in such as manner as to scanthe cylindrical ablations across the optical correction zone to theother side of the annular transition zone.

[0043] In addition, it should be noted that the cylinder width may bemaintained constant during the ablation procedure. In this embodiment,the displacement of the imaging lens 51 only provides the increasedcurvature on the corneal surface.

[0044] During the calculation of the positions of the offset mechanism,the actual laser pulse number is preferably mapped to a modified pulsenumber to produce positions of the offset mechanism that create auniform ablation on the eye during any point in the treatment. The sortalgorithm is specified by the number of layers that the completecylinder ablation should be divided into. In one embodiment, the pulsesfrom the two halves of the eye are arranged so that the offset motionstarts at one side and moves continually across the eye to the otherside. The pulses then reverse direction and move back to the originalside. Each pass of the offset mechanism comprises a layer. The entireprocedure will typically comprise about 5 to 15 layers, and preferablyabout 10 layers.

[0045] By separating the overall treatment into layers, motion of themechanical elements within each particular layer can be optimized. Also,in the event of an interruption in the treatment before completion, thepatient will be left with a partially completed ablation pattern whichwill be easier to align when the procedure is resumed or which isoptically beneficial if the procedure cannot be resumed.

[0046]FIG. 4 is a schematic view of the delivery system optics in thepreferred embodiment. As seen in this figure, the beam from laser 28 isreflected by a first mirror 71 and enters a spatial and temporalintegrator assembly 73, where the beam is modified in cross-section.Alternatively, the delivery optics may include a dove prism rather thana temporal beam integrator. The modified beam exiting from spatial andtemporal integrator 73 is reflected by mirror 74 and passed through alens 76 that collimates the beam, and through an iris/slit mechanism 78which contains the variable width slit and variable diameter irisdescribed above. The profiled beam exiting from the unit 78 enters theimage offset control unit 80 which contains imaging lens 51. The offsetprofiled image exiting from unit 80 is reflected from a mirror 82 ontothe patient's eye.

[0047]FIGS. 5 and 6 illustrate the image offset control unit 80. Asshown, imaging lens 51 is contained in a fixture 81, which is mountedfor pivotal motion about a first pivot post 83. Pivot post 82 is mountedin the internal recess of a fixture housing 87. A first drive motor 93is mounted to fixture housing 87 for rotating imaging lens 51 aboutpivot post 83. In the representative embodiment, drive motor 93comprises a rack and pinion drive with an arc shaped rack 94 thatengages teeth (not shown) for rotating lens 51. First drive motor 93provides rotational movement to lens 51 to vary the angle of lens 51,thereby changing the direction that lens 51 is translated. A seconddrive motor 89 is mounted on a flange portion 90 of housing 87 and hasan output shaft 91 for driving a second drive belt 92 which is coupledto the lower portion of housing 87.

[0048] In operation, when fixture 81 is driven by motor 93, the lens 51pivots about post 83. Similarly, motor 89 and belt 92 pivot housing 87about flange 90 and base 92. By operating motors 89, 93 simultaneously,compound motion of fixture 81 can be effected so that both translationaland rotational motion can be imparted to the lens 51. For example, ifthe rotational movement of lens 51 about post 82 is offset by therotational movement of the entire fixture housing 87, purelytranslational movement of lens 51 occurs. Motors 89 and 97 are driven bythe computer 21. By properly programming computer 21, the desired motioncan be imparted to imaging lens 51 in order to scan the aperture imageover the desired ablation region of the corneal surface. An alternativeoffset imaging mechanism is described in U.S. patent application Ser.No. 08/058,599, filed May 7, 1993 for “METHOD AND SYSTEM FOR LASERTREATMENT OF REFRACTIVE ERRORS USING OFFSET IMAGING”, the completedisclosure of which has previously been incorporated herein byreference.

[0049] The invention affords great flexibility in performing varioustypes of corrections by virtue of the fact that the system can beprogrammed to accommodate patients having differently sized physical eyeparameters and refractive correction requirements. The slitwidth/variable diameter iris arrangement is particularly adaptable foruse in the treatment of hyperopic astigmatism. For simultaneoustreatment of hyperopia and astigmatism, the ablation geometry is solvedas a function of image lens displacement and variable aperture size, asdiscussed above. Further, in all procedures requiring a smoothing of thetransition zone at the periphery of the ablation zone, the diameter ofthe iris is varied over a predetermined range. For refractiveaberrations, a device such as a spatially resolved refractometer or atopography machine or both may be used to map the irregular surfacecontour of the cornea to determine the exact surface correctionsrequired. Thereafter, the slit width and the iris diameter can beprogrammed such that corneal sculpting will achieve the desiredcylindrical surface geometry in the optical correction zone.

[0050] In addition to hyperopic corrections, the invention can be usedfor other visual error corrections, both regular and irregular, forphototherapeutic keratectomy (typically used to ablate scar tissue), andfor smoothing ablations. For phototherapeutic keratectomy applications,a scar which occurs centrally over the cornea can be ablated with theexcimer laser by ablating a large area with a transition zone at theedge. As in the case with astigmatism and hyperopia, it is desirable toposition the transition zone as far from the optically used portion ofthe cornea as possible. This avoids potentially undesirable side effectsof scar removal, such as hyperopic shift in which changes in thecurvature of the cornea create a hyperopic condition.

[0051] For any of the above specific correction procedures, a treatmenttable is normally constructed containing the value of all of thediscrete radial positions of the optical-mechanical elements used toscan the image over the relevant portion of the anterior cornealsurface, as well as the number of laser pulses per position. A typicaltreatment table contains on the order of about 500 different entries.

[0052] The treatment table for a given procedure may incorporate specialfeatures designed to improve the efficiency of the procedure. Forexample, for some procedures (e.g., hyperopic correction) it can bebeneficial to leave a small zone centered on the optical zone untreated.This can be done by constraining motion of the inner cylinder blade toguarantee occlusion in the small zone of interest. Further, compensationfor variable or differential healing rates and for differential ablationdepth due to tissue hydration may be factored into the treatment table.

[0053] While the invention has been described above with specificreference to ablation of the anterior corneal surface, other portions ofthe cornea may also be treated using the invention. For example, theepithelium may be mechanically removed by scraping, as is typically donein photorefractive keratectomy, and the exposed surface may be ablated.Further, the invention can also be used for laser keratomileusis ofcorneal lamella removed from the cornea. This procedure is described inU.S. Pat. No. 4,903,695 issued Feb. 27,1990 for “METHOD AND APPARATUSFOR PERFORMING A KERATOMILEUSIS OR THE LIKE OPERATION”. In applying theinvention to this procedure, a flap of corneal tissue is physicallyremoved from the cornea, the size of the removed portion typically lyingin the range from about 8 to 10 mm wide and a variable thickness up to250 microns. This flap of tissue is typically removed using amicrokeratome. Next, the flap is placed in a suitable fixture—typicallyan element having a concave surface—with the anterior surface face down.Thereafter, the required ablation is performed on the reverse exposedsurface of the flap, after which the ablated flap is repositioned on thecornea and reattached by suturing. Alternatively, after the flap isremoved from the cornea, the exposed stromal tissue of the eye can beablated according to the invention, after which the flap is re-attachedover the freshly ablated stromal tissue. In other procedures, the flapis folded away from the rest of the corneal instead of being entirelyremoved from the cornea. In these procedures, the ablation is performedon the exposed stromal tissue, and the flap is then folded back over andre-attached to the freshly ablated stromal tissue.

[0054] While the above provides a full and complete disclosure of thepreferred embodiments of the invention, various modifications, alternateconstructions and equivalents may be employed as desired. For example,while the invention has been described with specific reference to thesystem of FIGS. 1 and 2, other systems may be employed, as desired. Forexample, the systems and methods described herein may be employed inconjunction with the T-PRK® scanning and tracking laser from AutonomousTechnologies Corporation, the SVS Apex laser from Summit TechnologyInc., the Keracor™ 117 scanning laser system from Chiron Vision, or thelike. Further, lasers of other appropriate wavelengths than laser 28 maybe used, if desired and effective. Also, laser beam systems whichoperate on the principle of thermal ablations, such as lasers havingwavelengths lying in the infrared portion of the electromagneticspectrum, may be used to implement the invention. In addition, while theradial and angular positioning of the profiled beam is accomplished withimaging lens 51 in the preferred embodiment, other optical scanningelements—such as rotating mirrors and prisms—may be employed, ifdesired. Therefore, the above description and illustrations should notbe construed as limiting the invention, which is defined by the appendedclaims.

What is claimed is:
 1. A method of performing selective ablation of acorneal surface of an eye to effect a desired corneal shape, wherein thedesired corneal shape includes a first section having a first depth oftissue removal and a second section having a second depth of tissueremoval less than the first depth, the method comprising: establishingan optical correction zone on a corneal surface of an eye; directing alaser beam through an aperture to produce a profiled beam having across-sectional area smaller than the optical correction zone; anddisplacing the laser beam to a plurality of selected locations on theoptical correction zone to effect the desired corneal shape, wherein atleast a portion of a substantial amount of the selected locations coversthe first section of the optical correction zone to remove cornealtissue to the first depth at the first section, and at least a portionof a lesser amount of the selected locations cover the second section toremove corneal tissue to the second depth at the second section that isless than the first depth.
 2. The method of claim 1 wherein the firstsection comprises an outer boundary of the optical correction zone, andthe second section comprises a central portion of the optical correctionzone.
 3. The method of claim 2 wherein at least a portion of everyselected location covers the first section, and less than two of theselected locations cover the second section.
 4. The method of claim 1wherein the aperture comprises a slit such that the profiled beamproduces a cylindrical correction onto the corneal surface.
 5. Themethod of claim 1 further comprising the step of allowing the laser beamto pass through a variable diameter diaphragm and a variable width slit.6. The method of claim 5 further comprising selectively varying thediameter of the diaphragm and the width of the slit during thedisplacing step.
 7. A method of performing selective ablation of acorneal surface of an eye to effect a desired corneal shape, wherein thedesired corneal shape includes a first section having a first depth oftissue removal and a second section having a second depth of tissueremoval less than the first depth, the method comprising: establishingan optical correction zone on a corneal surface of an eye; directing alaser beam through an aperture to produce a profiled beam having across-sectional area smaller than the optical correction zone;displacing the laser beam to a plurality of selected locations on theoptical correction zone to effect the desired corneal shape, wherein atleast a portion of a substantial amount of the selected locations coversthe first section of the optical correction zone to remove cornealtissue to the first depth at the first section, and at least a portionof a lesser amount of the selected locations cover the second section toremove corneal tissue to the second depth at the second section that isless than the first depth; allowing the laser beam to pass through avariable diameter diaphragm and a variable width slit; and reducing thediameter of the diaphragm and the width of the slit as the laser beam isdisplaced radially outward.
 8. The method of claim 7 further comprisingestablishing an annular transition zone outside of the opticalcorrection zone, the annular transition zone having an outer boundary,wherein the diameter of the diaphragm is selected such that an outeredge of the profiled beam remains between the optical correction zoneand the outer boundary of the transition zone.
 9. A system forperforming selective ablation of a corneal surface of an eye to effect adesired corneal shape, the system comprising: a laser for generating alaser beam; delivery optics coupled to the laser for directing the laserbeam onto a corneal surface of an eye; a variable aperture element forprofiling the laser beam to perform cylindrical corrections on thecornea; and a laser beam direction system for displacing the laser beamacross the cornea, wherein the laser beam direction system and thevariable aperture element operate in conjunction to change the cornealsurface from an initial curvature having hyperopic and astigmaticoptical properties to a subsequent curvature having correctivelyimproved optical properties.
 10. The system of claim 9 wherein thevariable aperture element comprises a variable width slit for generatingvariable width rectangular ablations on the corneal surface.
 11. Thesystem of claim 9 further comprising means for establishing an opticalcorrection zone on the corneal surface and an annular transition zoneoutside of the optical correction zone, wherein the variable apertureelement comprises a variable diameter aperture for smoothing a surfaceof the annular transition zone.
 12. The system of claim 9 wherein thelaser beam direction system comprises means for displacing an image ofthe variable aperture element across the corneal surface.
 13. The systemof claim 9 wherein the laser beam direction system comprises an imaginglens positioned between the variable aperture element and the eye, and atranslational motor for translating the imaging lens along a lineardirection so as to translate an image of the variable aperture elementacross the corneal surface.
 14. The system of claim 9 further comprisinga slit rotation motor for rotating the slit such that the rectangularablations have an orientation substantially equal to a desired axis ofcylindrical correction.